The present invention relates, in general, to a new process for the fabrication of submicron, single crystal silicon, movable mechanical structures, and more particularly to a simplified reactive ion etching (RIE) process which is independent of crystal orientation and which produces controllable vertical profiles in the silicon.
Recent developments in micromechanics have successfully led to the fabrication of microactuators utilizing processes which have involved either bulk or surface micromachining. The most popular surface micromachining process has used polysilicon as the structural layer in which the mechanical structures are formed. For such a polysilicon process, a sacrificial layer is deposited on a silicon substrate prior to the deposition of the polysilicon layer. The mechanical structures are defined in the polysilicon, and then the sacrificial layer is etched partially or completely down to the silicon substrate to free the polysilicon movable mechanical structures. The present polysilicon technology is not easily scaled for the formation of submicron, high aspect-ratio mechanical structures, because it is difficult to deposit fine-grain polysilicon films to the required thickness.
Some bulk micromachining processes can yield mechanical single crystal silicon structures using wet chemical etchants such as EDP, KOH, and hydrazine to undercut single crystal silicon structures from a silicon wafer. However, such processes are dependent on crystal orientation within the silicon wafer, and for this and other reasons the type, shape and size of the structures that can be fabricated with the wet chemical etch techniques are severely limited.
The use of single-crystal materials for mechanical structures can be beneficial, since these materials have fewer defects, no grain boundaries and therefore should scale to submicron dimensions and retain their structural and mechanical properties. Also, the use of single-crystal materials, particularly single crystal silicon and gallium arsenide, to produce mechanical sensors and actuators can facilitate and optimize electronic and photonic system integration. For example, single crystal silicon (SCS) structures having a very small mass, in the range of 2.times.10.sup.13 kgm, can resonate without failure at 5 MHz for 2 billion cycles with a vibrational amplitude of plus or minus 200 nm. Accordingly, the fabrication of submicron mechanical structures with high aspect ratios is highly desirable.